BUG: (GH3795) better error messages for invalid dtype specifications in read_csv

ENH: accept 'str' as a dtype in read_csv to provide correct parsing

closes #3795
```
In [1]: df = DataFrame(np.random.rand(5,2),columns=list('AB'),index=['1A','1B','1C','1D','1E'])

In [2]: df
Out[2]: 
           A         B
1A  0.096563  0.761440
1B  0.623102  0.538810
1C  0.498820  0.277789
1D  0.113544  0.723437
1E  0.381104  0.061758

In [3]: df.dtypes
Out[3]: 
A    float64
B    float64
dtype: object

In [4]: path='test.csv'

In [5]: df.to_csv(path)

In [6]: pd.read_csv(path, dtype=str, index_col=0)
Out[6]: 
                      A                    B
1A  0.09656290409114332    0.761440208545324
1B   0.6231015058315575   0.5388097714651147
1C  0.49881957371373464  0.27778943212477014
1D  0.11354443109778356    0.723437196012621
1E  0.38110436826261596  0.06175758774696094

In [7]: pd.read_csv(path, dtype=str, index_col=0).dtypes
Out[7]: 
A    object
B    object
dtype: object
```

Invalid dtype specifciation, all ``TypeError``, slightly different messages depending on what you are doing (e.g. if its completely invalid or really a date spec)
```
In [8]: pd.read_csv(path, index_col=0, dtype={'A' : 'foo'})
TypeError: data type "foo" not understood

In [9]: pd.read_csv(path, index_col=0, dtype={'A' : 'datetime64'})
TypeError: the dtype <M8 is not supported for parsing, pass this column using parse_dates instead

In [10]: pd.read_csv(path, index_col=0, dtype={'A' : 'timedelta64'})
TypeError: the dtype <m8 is not supported for parsing

```